The present invention relates to a semiconductor memory device, and more particularly to a semiconductor memory device which overdrives for a predetermined period when sense amplifying a bitline and a bitline sense amplifying method of the same.
In general, a semiconductor memory device uses overdriving of a bitline sense amplifier for improving a row access strobe (RAS) to column access strobe (CAS) delay time (tRCD), which is an AC property of the core.
A wordline is enabled when an active command is applied, and cell data is thus loaded on a bitline. Charge sharing occurs and the difference in potential of a bitline pair BL, BLB has a predetermined level, as is shown in FIG. 1.
Thereafter, the bitline pair BL, BLB is amplified to the levels of a core voltage VCORE and a ground voltage VSS respectively by a bitline sense amplifier (not shown). At this time, the overdriver is used to improve tRCD.
During the enabled period of an overdrive enabling signal ODEN the overdriver shorts the core voltage VCORE terminal and the power voltage VDD terminal to supply a higher level voltage than the core voltage VCORE to a pull-up node of the bitline sense amplifier.
The overdrive enabling signal ODEN sets the operation period of the overdriver and is typically generated using an overdriving pulse generating circuit having the configuration shown in FIG. 2.
As shown in FIG. 2, the conventional overdriving pulse generating circuit includes a delaying/inverting unit 20, which delays and inverts a sense amplifier enabling signal SAEN; and a combining unit 22, which logically combines the output of the delaying/inverting unit 20 with the sense amplifier enabling signal SAEN to output the overdriver enabling signal ODEN.
A typical overdriving pulse generating circuit having the above described configuration generates an overdriver enabling signal ODEN having a predetermined pulse width using the sense amplifier enabling signal SAEN. At this time, the pulse width of the overdriver enabling signal ODEN is the extent of the delay created by the delaying/inverting unit 20.
However, the extent of the delay created by the delaying/inverting unit 20 varies with the external environment, for example the process, voltage, and temperature (PVT).
Particularly, when the pulse width of the overdriver enabling signal ODEN becomes larger than the target due to an increase in the extent of the delay, the time in which the core voltage VCORE terminal and the power voltage VDD terminal are shorted is lengthened. As a result, the level of the overdrive voltage is increased to a level more than that which is needed. Accordingly, there is a problem, in that the potential of the bitline is higher than the core voltage VCORE even during normal driving of the bitline sense amplifier, and thus normal precharge is not carried out during precharge, thereby causing defects.
Additionally, the pulse width of the overdriver enabling signal ODEN becomes smaller than the target when the extent of the delay is decreased due to the external environment. In this case, there is a problem, in that the tRCD property becomes inferior, since overdriving of the bitline sense amplifier is not carried out sufficiently.
Meanwhile, as shown in FIG. 1, a column selection signal YI is enabled after a predetermined time from the completion of overdriving, and the bitline pair BL, BLB and a local data line pair LIO, LIOB are connected.
At this time, the local data line pair LIO, LIOB is precharged to a level lower than the core voltage VCORE (typically, ½ VCORE), and therefore a drop in the voltage level occurs in the bitline BL, which has the level of the core voltage VCORE when the bitline pair BL, BLB and the local data line pair LIO, LIOB are connected.
Particularly, if there is a large drop in the level of the bitline voltage or if the bitline does not recover sufficiently within a predetermined time according to a minimum RAS to CAS delay time (tRCD_min) condition, the local data line pair LIO, LIOB may not have a sufficient difference in potential.
When this occurs, the sensing operation of the data sense amplifier 10, which sense amplifies the potential difference of the local data line pair LIO, LIOB, is either not carried out normally or delayed. Accordingly, a problem results, in that data is delivered late to a global data line GIO, and defects in subsequent operation occur.